Ultrasound imaging systems are used in medicine to explore internal areas of a patient's body. Ultrasonic imaging is non-destructive and versatile and can provide high quality diagnostic images.
A typical medical ultrasound imaging system has a transducer, a custom built electronic controller, and a user interface. The transducer typically has several piezoelectric transducer elements regularly placed on a frame. The transducer may have elements arranged in any of several different geometries, depending upon the medical application for which the transducer will be used. The controller drives the transducer and collects and processes data from the transducer to provide, store, display and manipulate images. The user interface may include various input/output devices which allow a user to control the operation of the imaging system. The input/output devices typically comprise at least a control panel, a video display, and a printer.
The electronic controller can send and receive electric signals to and from any of the transducer elements. To create a diagnostic image, the controller transmits electrical excitation signals to the transducer elements. The transducer elements convert the excitation signals into ultrasonic vibrations which are transmitted into the patient's body. The ultrasonic vibrations typically have frequencies in the range of about 2 MHz to about 12 MHz. The ultrasonic vibrations are scattered and reflected by various structures in the patient's body. Some of the reflected and/or scattered ultrasonic vibrations, which may be called echoes, are received at the transducer. The echoes cause the transducer elements to generate electrical signals. After the excitation signals have been transmitted the controller receives and processes the electric signals from the transducer elements.
The resulting image is displayed in real time on a display. The classic presentation of the display, called B-mode, is a two-dimensional image of a selected cross-section of the patient's body. It is desirable to obtain high resolution images and so state of the art medical ultrasound imaging system provide dynamic digital focusing and adaptive filtering systems which process the signals received from numerous transducer elements. Another commonly used presentation, called M-mode, shows a particular line of the image, displayed as a function of time on the screen. M-mode images are useful for displaying temporal information.
To provide more diagnostic information, ultrasound imaging systems typically include digital signal-processors which process the echoes to determine blood flow velocity at points in the patient's body. The blood flow velocity can be measured by using Doppler processing techniques to extract the Doppler signal. The Doppler signal can then be processed by Fast Fourier Transform (FAT) techniques. The color of individual pixels in an image may be set to a value which indicates blood velocity at that point. A Doppler signal can also be extracted, processed by using auto correlation techniques to extract blood speed average and variance information in each point of a region of interest.
Commercially available medical ultrasound units typically have many custom made electronic boards which each perform specific tasks and which are hard wired together to provide desired imaging modes. This architecture has been considered necessary to achieve high quality ultrasound signal processing and display in real-time. Manufacturing, upgrading and maintaining such systems is undesirably expensive. Since these systems use their own standards, it is also more expensive to develop user software, and more expensive than desirable for customers to acquire network and archiving systems compatible with the ultrasound imaging system.
U.S. Pat. No. 5,758,649 to Iwashita et al. describes a system for generating ultrasound images. The system includes a module which performs analog beam-forming, preprocessing, scan image conversion and display. This module is designed to be used in conjunction with a computer. The module can be interfaced to a networked general purpose computer system. Images generated by the module can be displayed on the computer. The ultrasound imaging system described in Iwashita et al. uses hard wired electronic boards for preprocessing, scan conversion, and post-processing.
U.S. Pat. No. 5,839,442 to Chiang et al. describes a portable ultrasound imaging system comprising a computer and a miniaturized analog beam-former integrated in a transducer. This system also uses application-specific hard wired electronic boards to perform signal preprocessing. The scan conversion and display software described provides only restricted post-processing.
U.S. Pat. No. 5,795,297 to Daigle describes an ultrasound imaging system comprising a beam-former and a personal computer. The computer is responsible for almost all processing except beam-forming. The Daigle system is not capable of performing in real time many of the more sophisticated tasks which are now expected of diagnostic ultrasound systems. For a typical digital ultrasound real-time imaging system, implementing a 128 channel digital beam-former needs about 500,000 MIPs (million instructions per second), an echo-level preprocessor needs about 600 MIPs, a color Doppler preprocessor needs about 2,000 to 5,000 MIPs and a scan converter needs about 150 MIPs. Currently available personal computers and workstations can typically handle 200 to 1,000 MIPs with added digital signal-processing boards. This is sufficient to perform scan conversion in real-time. However, echo-level preprocessing with color Doppler preprocessing cannot be performed in real-time on such platforms. Furthermore, presently available real-time operating systems for personal computers are capable of interrupting in a minimum of 2 milliseconds. This makes it necessary to have a hardware controller for the transmit and receive sequencing.
There remains a need for a flexible, versatile and programmable ultrasound imaging system. There is a particular need for such systems which are based on readily available computers but which can provide sophisticated digital real-time imaging.